The present invention relates to a tubular heat exchanger and to a process for its manufacture.
It is known, in the recovery of energy from a medium, say a gaseous one like an exhaust gas, to make use of another medium, for example fresh air, and also to utilize tubular heat exchangers comprising a plurality of parallel tubes made from industrial silicates, for instance glass. The tubes form the flow path for one of the media, while the flow path for the other medium is formed by the gaps between the tubes.
Known heat exchangers of this kind comprise a metal housing with two opposite metal plates with bores for inserting the tubes. The tubes are held in said bores by elastic sealing sleeves ensuring that the tubes are both tightly and elastically mounted to said plates. The use of the seals and the tubes requires a complex manufacturing process. Since force is required to insert the tubes, once the seals have been mounted to the plate bores, there is an appreciable danger of breakage. Also, the tightness of every individual plate bore is not ensured.
An object of the present invention is to provide an improved tubular heat exchanger and a method for its manufacture.
Another object of the present invention is to provide a tubular heat exchanger wherein the end portions of the heat exchanger tubes making up the heat exchange are elastically sealed together, thereby providing a tight elastic support for the tubes.
A further object of the present invention is to provide a tubular heat exchanger which places little stress on the ordinarily thin-walled tubes.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention proposes a heat exchanger which not only eliminates the complex insertion of the tubes into narrow plate bores provided with sealing sleeves, but also provides, by simple manufacturing steps, the flawless, tight and elastic support for the tubes. The tubular heat exchanger of the present invention achieves this end by providing two opposite and spaced-apart metal beams, each adhering to a wall of a hardened, elastic plastic, the end portions of said tubes being firmly and adheringly embedded in said hardener. The tubes are made of an industrial silicate.
The hardened, elastic, plastic material, for instance silicone-rubber, is used not only because of its flawless surface adhesion for sealing the tube passages, but also because of its elasticity which provides a firm, yet yielding support for the tubes so that they are protected against breakage from impact or vibration when moving and assembling the heat exchanger and when the heat exchanger is in operation.
The process for manufacturing the heat exchanger according to the present invention is characterized in that in the region of the end portions of the tubes which are spaced substantially parallel to one another and temporarily supported, a wall made by casting plastic is built up for enclosing the tube ends, whereupon the material hardens but remains elastic.
The building of the walls may take place vertically by alternating the deposition of the viscous material and a layer of tubes in an upright frame, or horizontally by pouring the elastic hardening material around the tubes supported by a base in a reclining frame. A process which was found to be especially advantageous mounts and dimensions the holes corresponding to the tubes in a foil or the plate disposed in the frame. The foil or plate may be made of plastic or metal sheet and said holes extending into a standing tube bundle are temporarily fixed with respect to the same, whereupon the frame is cast or "potted" with liquid plastic. The liquid plastic penetrates into the gaps between the tubes and the bore in the plate or foil by capillary action. It was found that this necessarily leads to centering of the tubes in the plate or foil holes. This ensures that the plastic surrounds the tube near the plate or foil very evenly, that is, with nearly a constant layer thickness, so that upon hardening of the plastic, each tube is elastically held on all sides. Building up the wall from a liquid plastic not only is relatively easy to carry out, but in addition, provides an absolutely tight joint between the tube and the wall and flawless support of the tube at the wall.
If the process is to be carried out for horizontally disposed tubes, that is, by a layerwise build-up of the tubes, then two horizontal, parallel bars are coated with a layer of viscous adhesive which is capable of hardening into an elastic seal, and a first layer of tubes disposed in a spaced-apart relationship are pressed by their ends into the adhesive layers. Additional adhesive layers are then deposited over the first layer of tubes and each layer of tubes is, in turn, pressed into the adhesive layers. Bars which are identical to the lower ones are pressed into the adhesive strips of the top layer, thus covering the last layer of tubes. The adhesive strips from each side of the tubes connect with each other to form a wall which, upon hardening, adhere tightly and elastically to the bars and to the tubes. The heat exchanger units thus comprise a pair of walls, each lying between a pair of bars with the tubes tightly adhering therein being inserted into the inlets and outlets of the flow paths of the housing for the two media.
As already mentioned, the casting method has been found to be especially advantageous. It allows for making heat exchangers wherein the supporting tube walls are made from an elastic plastic and do not require a rigid insert plate or foil. Thus, this method allows for placing the tubes between a lower and an upper template, sealing the tube ends with respect to the outside and determining the tube separations, one mounting frame being mounted to each template, whereupon after rotating the unit by 180.degree., the other pan traversed by tubes and bounded by the template and mounting frame is potted with an adhesive hardening into an elastic solid, whereupon the templates are removed. Silicone rubber again may be used in this case as the adhesive. If the heat exchanger is intended to be subjected to large requirements regarding pressure and temperature differences, for example for heat exchanges utilizing liquids, a rubber similar to natural rubber, e.g., butadiene-styrene, and subjected to a vulcanizing process will be appropriately used as the adhesive.
For relatively large tube separations, a template again may be positioned prior to potting into the potting dish, the bores allowing for radial play when the tubes are inserted. Only the interstices remaining between the tubes and the template require potting. The template then may itself serve as the frame or it may be potted together with one.
It was found in many cases that the chemical resistance of the adhesive will sometimes be insufficient. Therefore, it was found to be particularly appropriate to cover that side of the wall formed by the adhesive and which is particularly exposed to the reactive media with a protective foil, for example, Teflon. A hole-bearing protective foil corresponding to the array of tubes is deposited on the bottom of the template-part forming the potting dish. This protective foil remains as a lost sheathing in the finished exchanger structure and covers the particular sidewall of the adhesive and protects it from the contacting medium.
It was found for heat exchangers with relatively long tubes that vibrations may occur especially in operating with gaseous media of high flow rates, and thus tube breakage may result. In such cases, it may therefore be necessary to additionally support the tubes between their two support walls so as to prevent the undesirable, strong vibrations. This additional support may be secured by interposing rods between adjacent tube layers or by providing intermediate template-like support foils or walls which are traversed by the tubes. Such support walls may basically be of the same design as the lateral support walls. In such a case the intermediate walls enclosing and tightly adhering to the silicate tubes divide the flow channel between the tubes into a corresponding number of sections.
Not only are all of the manufacturing processes of the present invention easy to carry out, but also they necessarily lead in the same operational sequence to tightly, elastically and yieldingly fix the tubes into the wall formed from the adhesive.
Since it is immediately possible to equip the heat exchanger with two, four or more such units all equal to one another, the individual heat exchanger made up of the same units can, despite mass production, meet or be adapted to meet practically all existing requirements regarding size or output.